var sorc="img/bg1440.jpg";
if (screen.width<=800)  
    {sorc="img/bg800.jpg";}  
  else  
 if (screen.width<=1024)  
    {sorc="img/bg1024.jpg";}  
  else
 if
    (screen.width<=1280)  
    {sorc="img/bg1280.jpg";} 
 else
 if
    (screen.width<=1440)  
    {sorc="img/bg1440.jpg";}
function graphfon(kartinka) {
	document.images[kartinka].src=sorc;
	}